Currently it is possible to get the solar position of arbitrary planets (and therefore also the position of a planetary body relative to its parent in the case of moons). However it is not always possible to get the orbital velocity vector of a planet. Specifically it is necessary to target the planet in order to get it's orbital velocity via the velocity [target] expression. Unfortunately it is not possible for a craft to target it's parent, thereby making it impossible to get its orbital velocity.
The reason I want to get my parent planet's orbital velocity is so I can calculate it's Keplerian orbital elements, so I wouldn't complain if I could fetch those as well or instead (per this suggestion).
@theUSSR another workaround would be to get solar position, wait one second and then take the difference between current solar position and the previous position. However I'm not sure how precise this would be.